1. Field of the Invention
The present invention pertains to a method for the purification of 4-acetoxystyrene. In particular, the method comprises at least one crystallization without the aid of a solvent.
2. Background of the Invention
The product 4-acetoxystyrene, which is typically obtained through the elimination of water from 4-acetoxyphenylmethyl carbinol, represents an important intermediate from which a variety of compounds are produced. The 4-acetoxystyrene is used as a monomer for the manufacture of poly(4-acetoxystyrene) from which poly(4-hydroxystyrene) and various copolymers are produced. In addition, 4-acetoxystyrene can be used to prepare 4-hydroxystyrene from which numerous derivatives having medical applications are produced. Polymers and copolymers of the kind described above are applied, for example, in the electronic industry as a binder in photoresists, in adhesives to improve their temperature stability, and in the surface treatment of metals, for the replacement of toxic chromate as a subcoating on the metals. The poly(4-hydroxystyrene) polymers and copolymers are also used as a UV absorber and flame-retardant additive that cannot be extracted from the plastic and as an additive in other polymers such as polyesters, nylon, polyurethanes, etc. in order to substantially improve their adhesive properties to glass and mineral fillers.
According to the state of the art, 4-acetoxystyrene, which is normally contaminated by starting reaction components such as 4-acetoxyphenylmethyl carbinol as well as by other compounds such as, but not limited to, acetic acid, ethyl benzene, 1-phenyl ethanol, 4-ethyl phenol, 4-hydroxystyrene, 4-ethylphenyl acetate, 4-acetoxyacetophenone, 1-(4-acetoxyphenyl)-ethane, heavy ends and water, has typically been purified by rectification. Since, interalia, the rectification is carried out at reduced pressure, the boiling temperatures of 4-acetoxystyrene and impurities such as 4-hydroxystyrene and 4-ethylphenyl acetate lie close together. Thus, a large number of separating stages are required, which subjects the mixture of reactants to high thermal stress. This is particularly a problem, since both 4-acetoxystyrene and the 4-acetoxyphenylmethyl carbinol reactant possess only limited thermal stability. As a result, a considerable amount of the 4-acetoxystyrene and the 4-acetoxyphenylmethyl carbinol are destroyed during rectification by subsequent reactions such as resinification, the formation of oligomers and also by polycondensation and polymerization. The process yield for conversion of 4-acetoxyphenylmethyl carbinol to 4-acetoxystyrene is reduced accordingly. In addition, this limited thermal stability also prevents the recycle of unconverted 4-acetoxyphenylmethyl carbinol, again reducing the potential yield of the process.
It was, therefore, the object of the present invention to provide a process for the preparation of pure 4-acetoxystyrene from crude mixtures, which process works under low thermal stress conditions and provides a high efficiency separation, leading to good yields of 4-acetoxystyrene having high purity.
Applicants' initial attempts to separate the valuable 4-acetoxystyrene from the multi-component mixture with the help of other, less thermally destructive methods, such as extraction, supercritical extraction, or adsorption, failed because it was not possible to find a sufficiently selective extraction or adsorption agent.